As shown in Japanese Patent Early Publication 57-23773, a typical open show-case having an inner and an outer air ducts provided independently in an inner and an outer boxes, respectively, of the show-case includes two pairs of an evaporator and a blower, one for each duct for circulating for desired refrigeration of the foods in a storage chamber.
In a refrigeration system including such evaporators there are provided, along with decompression elements connected with the evaporators, switching means for selecting the operative evaporators.
Usually, such switching means as mentioned above periodically switches the passage of the refrigerant such that the two evaporators alternately functions to cool the show-case while the remaining one is subjected to defrosting operation.
For example, in order to perform refrigeration by a first evaporator or the evaporator for the inner duct while defrosting a second evaporator or the evaporator for the outer duct, refrigerant is passed through the second non-operating evaporator to provide the sensible heat of the refrigerant to the second evaporator to melt the frost thereon. While passing through the second evaporator, the refrigerant is super-cooled, which is then passed through a first decompression element of the first evaporator, where the refrigerant is evaporated. In the opposite case where the refrigeration is performed by the second evaporator while defrosting the first evaporator, the passage of the refrigerant is switched so that it is first passed through the first evaporator, and the super-cooled refrigerant is then passed through the second evaporator.
The prior art system as mentioned above, however, has a disadvantage in that during refrigeration by one evaporator the other must be constantly supplied with refrigerant having temperature in the range of 30.degree.-35.degree. C. This results in cold air in one duct and warm air in the other duct, the latter air having temperature normally warmer than ambient air temperature. This is also the case in a double layered air curtain formed across the front opening of the show-case. That is, one layer of the air curtain has a higher temperature and the other lower temperature than the ambient air. Consequently, both of them are "neuteralized" in temperature, so that the temperature of the air curtain remains in the neighborhood of 10.degree. C., which is considerably higher than desired cooling temperature of 0.degree. C. (or less) in the storage chamber.
This problem is serious particularly when the second evaporator (for the inner duct) is undergoing defrosting, since then such a warm air curtain (warmer than the ambient air) is formed inside the cold air curtain, causing the temperature of the storage chamber to rise appreciably than desirable refrigeration temperature.